The Sukhoi Su-24 (NATO reporting name: Fencer) is a supersonic, all-weather attack aircraft developed in the Soviet Union. The aircraft has a variable-sweep wing, twin-engines and a side-by-side seating arrangement for its two crew. It was the first of the USSR's aircraft to carry an integrated digital navigation/attack system. Work on upgrading the Su-24 was started in 1971, and included the addition of inflight refueling and expansion of attack capabilities with even more payload options. T-6M-8 prototype first flew on 29 June 1977, and the first production Su-24M flew on 20 June 1979. The aircraft was accepted into service in 1983. Su-24M has a 0.76 m (30 in) longer fuselage section forward of the cockpit, adding a retractable refueling probe, and a reshaped, shorter radome for the attack radar.